Kobo e-ink based ereaders such as the Aura run Linux and have never mounted as MTP. The should be mounted as a USB MSD. Or whatever the MAC calls that. There have been issues with new MACs, due to changes in how they work. I believe they have been sorted out, but, don't know enough to tell you what they are.
But, a starting point is to do what is in
https://www.mobileread.com/forums/sh...d.php?t=118560. Posting the results of the last step will let us see what calibre is actually seeing connected to the device.